Pakistan's worst floods in decades have left millions hungry, the United Nations said on Monday, while parts of the south were on high alert for rising waters that could further tax aid groups. "We cannot talk about starvation yet but I think we can talk about millions of people being hungry," Maurizio Giuliano, the U.N. humanitarian spokesman, told Reuters. "I think we have millions of people who are hungry, and hunger is clearly a factor that contributes significantly to vulnerability." The flood has been spreading through the rice-growing belt in southern Sindh province district by district, breaking through or flowing over embankments. Waters have been rising in southern Sindh and hundreds of thousands of people have fled cities, towns and villages for safer ground, disaster management officials said. Sindh is home to Pakistan's biggest city and commercial center Karachi, but the floods have affected mostly rural areas and far smaller urban centers. Over 100,000 people have fled the Sindh city of Shahdadkot, and officials say one of their biggest concerns now is growing water pressure in the Indus River along the southern cities and towns of Hyderabad, Jamshoro and Thatta which could lead to more flooding. Saleh Farooqui, head of the Sindh Provincial Disaster Management Authority, said over 100,000 people have been evacuated from Thatta alone. The worst floods in decades have destroyed villages, bridges and roads, made more than 4 million homeless and raised concerns that militants will exploit the misery and chaos. The government has been accused of moving too slowly and Islamist charities, some with suspected links to militant groups, have moved rapidly to provide relief to Pakistanis, already frustrated with their leaders' track record on security, poverty and chronic power shortages. More than $800 million has been donated or pledged to help Pakistan's flood victims, the foreign minister said on Sunday. Long-term rebuilding will cost billions of dollars, pressuring a government that was already constrained by a fragile economy before one of the worst catastrophes in its history struck. The International Monetary Fund said it would review Pakistan's budget and economic prospects in light of the disaster in talks with government officials starting on Monday. The meetings in Washington will focus on a $10 billion IMF programme agreed in 2008, and the budget and macroeconomic prospects will be reviewed because of the magnitude of the flood disaster, officials said. Half a million people are living in about 5,000 schools in flood-hit areas of Pakistan. The cramped, unhygienic conditions, as well as the intense heat, raise the specter of potentially fatal disease outbreaks, such as cholera.